The present invention relates to synthetic aperture radar (SAR) systems, and more particularly to an SAR system having dead-ahead beam sharpening capability.
Synthetic aperture radar is well known for such applications as high resolution ground mapping. In this technique, the effective antenna aperture length is increased by utilizing the motion of the moving vehicle, such as an aircraft, in which the radar equipment is located. The effective antenna aperture length is increased by combining the return signals from a number of pulses transmitted by the radar equipment while the aircraft is moving relative to the ground area under scrutiny to form each azimuth line of the azimuth-range grid being processed. This technique permits very high azimuth resolution to be achieved.
A point in the radar map image can be located by two coordinates, the distance from the radar to the point and the amount of Doppler shift due to the platform motion of the radar signal for that point. The range can be measured by the time it takes for the radar signal to return to the SAR receiver. The measurement of the Doppler shift of the returned signals gives the azimuth location of the point with reference to the SAR's line of flight, since the moving vehicle carrying the SAR equipment will have a velocity component either approaching or receding from the point at any given time. If the antenna is not pointed in the direction of the velocity vector, each point in the radar beam will have a unique Doppler shift and range; these two coordinates and a knowledge of the angle of the antenna's line of sight with respect to the surface are all that are needed to get a fix on the location of any returned signal. Knowledge of the antenna pointing is required to resolve the ambiguity arising from points on the opposite side of the velocity vector which have the same range and doppler coordinate. The brightness of the image at that point becomes an element of the map image.
The conventional wisdom in the SAR art that it is impossible to provide beam sharpening in the direction of the velocity vector of the moving vehicle. This occurs because the antenna when pointed in that direction cannot isolate the points, i.e., resolve the ambiguities, on opposite sides of the velocity vector with the same range and doppler. As a result, SAR systems have not been used for many applications, such as aircraft landing systems, gunfire control radars and bombing radars, because the conventional synthetic array cannot look dead-ahead.